Illuminance Ev is the area density of luminous flux received by an illuminated body, integrated with all wavelengths and all directions. Illuminance is used to gauge the amount of light incident on a surface. A unit of illuminance is a lux (lx), lumens per square meter. Another unit of illuminance is foot candles. Illuminance on a surface area is luminous flux per area defined by the equation:
      E    v    =                    luminous        ⁢                                  ⁢        flux            area        =                  dΦ        v            dA      
The spectral illuminance Ev(λ) is defined by the illuminance per unit wavelength interval at the wavelength λ. Spectral illuminance Ev(λ) is related to the illuminance Ev by the equationEv=∫0∞Ev(λ)dλ. 
A device or application that measures illuminance is referred to as a luxmeter. Luxmeter software applications that measure illuminance are available for use with portable electronic devices that include a digital camera. The luxmeter software application is executed by a processor and the operating system software (e.g., APPLE iOS and GOOGLE Android) of the portable electronic device. The accuracy of the luxmeter software applications is often limited by the sensor and digital camera used in the portable electronic device. In addition, OS manufacturers may only provide incomplete application programming interfaces (API) to these sensors, or prohibit direct access by other applications to these sensors. Consequently, luxmeter applications typically have limited access to metadata of the digital pictures taken with a digital camera, if available. Typical metadata parameters produced by portable electronic device cameras that are associated with digital photographs are:                ExposureTime;        ShutterSpeedValue;        FNumber;        ApertureValue;        BrightnessValue;        ISOSpeedRatings;        
These metadata parameters are sometimes stored in registers as register settings. The metadata parameters ExposureTime, ShutterSpeedValue, FNumber, and ApertureValue are well defined because they follow standard photographic values. The parameter ExposureTime is related to the parameter ShutterSpeedValue through the mathematical equation:
  ExposureTime  =      1          2      ShutterSpeedValue      The parameter FNumber is related to ApertureValue through the mathematical equation:FNumber=√{square root over (2ApertureValue)}
The metadata parameter values of BrightnessValue and ISOSpeedRatings vary from camera to camera, such that they are difficult to use for a luxmeter application. As a consequence, luxmeters in applications are quite approximate or require an external calibration before use.
A number of references disclosing measurement of illuminance conditions were developed during the argentic film era, such as U.S. Pat. No. 3,972,626, and U.S. Pat. No. 3,973,266, when film based cameras were used to capture photos. Digital cameras revolutionized the structure of cameras, capturing pixels of images and storing them into flash memory. A number of analog optical mechanisms were replaced with digital computations. Accordingly, some inventions disclosed in patents, such as U.S. Pat. No. 5,737,648, were no longer relevant. New illuminance measurement methods were introduced for digital cameras.
U.S. Pat. No. 7,071,456 discloses a camera illuminance sensor to set lighting levels of I/O systems, such as keypads, and backlights for adjustable displays. U.S. Pat. App. Pub. No. 2012/0236173 similarly discloses adapting a camera user interface to environmental conditions. U.S. Pat. App. Pub. No. 2012/0236173 discloses use of a plurality of sensors to allow corrections for underwater conditions such as very cold conditions, very bright conditions, and very dark conditions. U.S. Pat. App. Pub. No. 2012/0236173 discloses addressing illuminance conditions of extreme brightness and extreme darkness, but does not make precise illuminance measurements.
U.S. Pat. No. 7,629,998 and U.S. Pat. App. Pub. No. 2007/0002143 disclose a method and apparatus for measuring the performance of a camera, including illuminance. A test environment is disclosed to better comprehend and verify the specifications of a camera and test their fitness. The disclosed approach to measure illuminance requires that additional sensors be used for the test environment.
U.S. Pat. App. Pub. No. 2001/0007470 entitled MEASUREMENT OF ILLUMINANCE CONDITIONS uses external light emitting diodes (LEDs), as well as color filters and photo sensors to measure illuminance. The LEDs are used to control the lighting conditions. The intensity of each main color in the illuminance light is measured by dedicated photosensors having corresponding color measurement bands. These color intensities are used to adjust signals originating from the charge coupled device (CCD) array of the photo image sensor. Using external light emitting diodes (LEDs) and color filters makes it more difficult to measure illuminance.
It is desirable to ease the measurement of illuminance so that it can be readily used.